128 THE ANIMAL AND THE VEGETABLE. 



are black. Their canals are often inhabited by 

 little Crustacea. Such are the leading points 

 of the account given of sponges in the fifth book 

 of the History of Animals. The statements 

 are very exact, and must have been the result 

 of careful observation and inquiry. The account 

 there given of the tunicated mollusks, animals 

 which in external form closely resemble sponges, 

 and, from their sluggishness, easily confounded 

 with them by the careless observer who neglects 

 the examination of their anatomy, affords in- 

 ternal evidence of the observations in both cases 

 having been made by Aristotle himself, when en- 

 deavouring to discover essential distinctions, by 

 which to fix the line of demarcation between 

 the animal and vegetable kingdoms. Since his 

 time many famous naturalists have sought to 

 solve the same problem — but in vain, for scarcely 

 two zoologists, even at the present day, hold 

 the same view on this disputed point. 



